Die-cushion apparatus of press machine

ABSTRACT

The object of the present invention is to provide a die-cushion apparatus of a press machine capable of performing a preliminary acceleration imparted to a die-cushion pad following the lowering of a slide with an exact timing and high precision and being improved so as to eliminate a pressure deviation between an actual pressure in a hydraulic cylinder and a preliminarily set aimed value at a portion near a lower dead point of the slide stroke. The die-cushion apparatus is provided with a pneumatic cylinder (6), a servo valve (40) for discharge pressure control and a hydraulic cylinder (7) to which a pressurized oil is made up by suction and constructed such that the make-up of the pressurized oil to the hydraulic cylinder (7) at the preliminary acceleration is performed by a pressurized oil makeup circuit (16) provided with a servo valve (22) controlled by a control unit body (14), and a control signal from a pressure control body (27) controlling the discharge pressure control servo valve (40) is corrected in response to the slide lowering speed and the press operation speed.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a die-cushion apparatus provided for a pressmachine and more particularly to a control apparatus for carrying out apreliminary acceleration and pressure control of a die-cushion.

BACKGROUND ART OF THE INVENTION

In a known art, a die-cushion apparatus provided for a press has astructure which elastically supports a die-cushion pad disposed near alower mold through a plurality of die-cushion pins and in which, when anupper mold lowering together with a slide abuts, through a work, againsta blank holder to thereby apply a press load to the die-cushion pad, apressure in a die-cushion cylinder supporting the die-cushion pad fromthe lower side thereof is discharged, and then the blank holder is alsolowered together with the upper mold by an amount corresponding to acushion stroke.

In the press die-cushion apparatus of the structure described above, alarge colliding noise is generated when the upper mold lowering togetherwith the slide abuts against the blank holder through the work.

For this reason, in a conventional technology, the die-cushion pad ispreliminarily lowered at a speed lower than a lowering speed of theslide to thereby reduce the noise generated by the collision of theupper mold with the blank holder and hence to prevent the life times ofthe upper mold and the blank holder from being shortened.

In the conventional preliminarily accelerating device, the loweringspeed is regulated by controlling a flow rate to a preliminarilyaccelerating hydraulic cylinder provided for the die-cushion by means ofa servo valve.

Namely, a crank angle of the press is detected and when the crank anglereaches a preliminarily set angle, the servo valve is opened by apredetermined angle to thereby start preliminary acceleration, and whenthe crank angle reaches an angle at which the upper mold abuts againstthe blank holder, the servo valve is closed. Thus, the lowering speedcontrol at the preliminary acceleration has been performed by anopen-loop control mode.

However, in a case where the lowering speed control of the die-cushionis carried out by the above conventional open-loop control, largedispersion is caused in the lowering speed, and in the case of a largespeed relative to the upper mold, such effect as the reduction of thecolliding noise cannot be expected.

Further, also in a case where the die-cushion is lowered with a fastspeed, not only a desired object cannot be achieved by the abutment ofthe upper mold against the blank holder after the preliminaryacceleration, but also the normal press formation is not performed,resulting in a cause of production of defective. In addition, thesetting of the angle at which the preliminary acceleration starts is tobe decided through repeated trial formations, so that much time isrequired for the regulation of the lowering speed, thus beinginconvenient.

Furthermore, in a conventional art, a die-cushion of a press utilizedfor drawing formation is composed of a hydraulic cylinder and apneumatic cylinder in which pressurized oil is supplied by suctionoperation and the cushioning function is attained by a dischargepressure control of both the cylinders.

A die-cushion unit to be numerically controlled (NC) is connected tothis hydraulic cylinder to thereby vary a cushioning capacity bycontrolling the servo valve for pressure discharge connected to ahydraulic cylinder in accordance with the crank angle of the press. Acontrol unit e for controlling the servo valve has a structure, as shownin FIG. 6, for controlling a servo valve d by detecting a pressure ofthe hydraulic cylinder a by a pressure sensor b, comparing the detectedactual pressure with a preliminarily set aimed value by a comparator cand outputting the thus obtained pressure deviation to the servo valve dby applying a constant gain to the pressure deviation.

However, in the press, the lowering speed of the press changes as shownin FIG. 8 in response to the crank angle, the lowering speed becomeszero at a lower dead point (crank angle of 180°), and the slide speedchanges in response to the operation speed of the press. Further, thecharacteristic feature of the servo valve d controlling the hydrauliccylinder a is non-linear, and accordingly, an object to be controlled bythe control unit becomes nonlinear.

For the reason described above, in the conventional control unit, theactual pressure becomes dull near the lower dead point as shown by acurve B in FIG. 7 with respect to a pressure instructed value (aimedvalue) shown by a curve A in FIG. 7, and a pressure difference betweenthe aimed value and the actual pressure value becomes large and thecontrol performance is degraded, thus being inconvenience.

SUMMARY OF THE INVENTION

This invention was conceived in consideration of the above matters andaims to provide a die-cushion apparatus of a press machine capable of,in order to weaken an impact of an upper mold to a blank holder througha work in abutment therebetween, carrying out a preliminary accelerationof a die-cushion pad following to the lowering of a slide with an exacttiming and high performance. This is the first object.

The second object of the present invention is to provide a die-cushionapparatus of a press machine being improved so as to substantiallyeliminate pressure deviation between an actual pressure in a hydrauliccylinder and a preliminarily set aimed value at a portion near a lowerdead point of a slide stroke.

To achieve the above first object, according to the first embodiment ofthe present invention, there is provide a die-cushion apparatus for apress machine including a pneumatic cylinder attaining a cushioningfunction and a hydraulic cylinder enabling and locking cushioningcapability, the die-cushion apparatus comprising a pressurized oilmakeup circuit for making up a pressurized oil to the hydraulic cylinderat a preliminary acceleration period of a die-cushion following alowering motion of a slide, a servo valve disposed to the pressurizedoil makeup circuit, and a control unit body, in order to control theservo valve, for calculating a crank angle for preliminary accelerationstarting and a crank angle for preliminary acceleration finishing inresponse to a preliminary acceleration stroke value, a drawing strokevalue and a press operation speed value inputted through an operationpanel, for comparing an aimed value inputted into a comparator withrespect to the respective crank angles outputted from a crank angledetector with data relating a die-cushion stroke outputted from adis-cushion stroke position detector and for controlling the servo valvein a feedback mode so as to eliminate deviation when the deviation iscaused between the aimed value and the data.

In order to achieve the second object of the present invention, there isprovided a die-cushion apparatus for a press machine comprising apneumatic cylinder attaining a cushioning function, a hydraulic cylinderhaving a pressure discharge line connected to a discharge pressurecontrolling servo valve to enable and lock a cushioning capability, apressure detector disposed to the pressure discharge line between thehydraulic cylinder and the servo valve for detecting an actual pressureof the hydraulic cylinder, and a pressure control unit for comparing incalculation the actual pressure detected by the pressure detector and apressure instruction value generated from a pressure generator and forcontrolling the servo valve so as to make coincident the actual pressurewith the pressure instruction value in response to the deviation betweenthe actual pressure and the pressure instruction value, the die-cushionapparatus further comprising means for detecting a lowering speed of aslide of a press and an operation speed of the press and outputtingsignals representing the detected lowering speed and the operation speedof the press and means for correcting an output signal from the pressurecontrol unit for controlling the servo valve in response to the detectedsignals.

According to the die-cushion apparatus of the present inventionincluding the above embodiments, since the acceleration speed of thedie-cushion can be precisely controlled so as to obtain an aimed valueoutputted with respect to every crank angle, the colliding noisegenerated in an abutment of the upper mold against the blank holder canbe effectively reduced and the drawing amount can be also ensuredexactly, thus preventing the defective from producing.

Moreover, since the preliminary acceleration starting angle and itsfinishing angle can be automatically calculated only by inputting thepreliminary acceleration stroke and drawing stroke of the die-cushionpad in accordance with the mold, any troublesome adjustment is notrequired, thus improving the maneuverability.

Furthermore, since the control signal outputted from the pressurecontrol unit controlling the pressure discharge servo valve byconsidering the slide lowering speed and the press operation speed,which are factors for the non-linearlity of the object to be controlled,as parameters for the hydraulic pressure control in the hydrauliccylinder, the servo valve can be controlled by the control signalcorrected so as to keep the pressure near the preliminarily set aimedvalue at a portion near the lower dead point at which the slide speedbecomes slow, that is, to make small the gain of the integrated circuit.Accordingly, the pressure control characteristic in the hydrauliccylinder can be remarkably improved so as to make small the dullness ofthe actual pressure at the portion near the lower dead point and theproduction performance of the product can be remakably improved.

The above and other objects, embodiments and advantages of the presentinvention will be made clear to persons skilled in the art from thefollowing descriptions and accompanying drawings showing preferredembodiments coinciding with the principle of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a schematic structure representing a firstembodiment of the present invention;

FIGS. 2 and 3 are views both for the explanatory of operation of thefirst embodiment;

FIG. 4 is a view showing a schematic structure representing a secondembodiment of the present invention;

FIG. 5 is a view for the explanatory of operation of the secondembodiment;

FIG. 6 is a view showing a schematic structure of a conventionalexample; and

FIGS. 7 and 8 are views for the explanatory of operation of theconventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, two typical embodiments of the present invention will bedescribed in conjunction with the accompanying drawings (FIGS. 1 to 5).

First, FIGS. 1 to 3 represent a first embodiment of the presentinvention. Referring to FIG. 1, reference numeral 1 denotes a pressmachine body, in which a lower mold half 2 is mounted on a bolster 1aand a blank holder 2a disposed to the peripheral portion of the lowermold 2 is elastically supported by a die-cushion 4 through a pluralityof die-cushion pins 3 disposed in the bolster 1a.

This die-cushion 4 comprises a die-cushion pad 5 supporting therespective die-cushion pins 3 from the lower side thereof, a pneumaticcylinder 6 supporting the die-cushion pad 5 and a hydraulic cylinder 7connected to the die-cushion pad 5 through a piston rod 7a.

The inside of the hydraulic cylinder 7 is divided into an upper chamber7₁ and a lower chamber 7₂ by means of a piston 7b mounted to the pistonrod 7a. A pressurized oil is supplied into both the chambers 7₁ and 7₂through a pressurized oil makeup circuit 16 including a hydraulic pump 8and a servo valve 22, and both the chambers 7₁ and 7₂ are communicatedwith each other through a logic valve 9₁.

Reference numeral 9₂ denotes a locking valve incorporated in a dischargecircuit of the upper and lower chambers 7₁ and 7₂.

A stroke position of the die-cushion pad 5 is detected by a die-cushionstroke position detector 11 and is inputted into a control unit body 14.

In the control unit body 14, there are inputted, by means of, forexample, an operation panel 15, a crank angle (γ) detected by a crankangle detector 12 connected to a main shaft (not shown) of the pressmachine body 1, a pressing speed (spm) detected by a rotation detector13 detecting rotation of a main motor (not shown) and drawing stroke 1band preliminary acceleration stroke 1a of a work W separately formed.

The data inputted from the respective detectors 12 and 13 and theoperation panel 15 are then inputted, through an I/O port 17, into acaluculation processing unit 48, in which a preliminary accelerationstarting angle θ a and finishing angle θ b (see FIG. 2) are calculatedin accordance with the data preliminarily stored in memory ROM and RAMand the calcualted data are then outputted to a comparator 19.

To the comparator 19 is inputted a present die-cushion position from thedie-cushion position detector 11 and the present die-cushion position iscompared with the aimed value outputted from the calculation processingunit 18. In this comparison, the deviation is outputted to a solenoid ofthe servo valve 22 as a serve valve opening degree instruction signalthrough a D/A converter 20 and a gain set circuit 21, therebycontrolling the servo valve 22.

The operation will be described hereunder.

A die-cushion motion is represented by a curve DC in FIG. 2 with respectto a slide motion shown by a curve SL. When the preliminary accelerationof the die-cushion is started with the crank angle θ a, the upper mold23 secured to the slide 10 abuts against the blank holder 2a with thecrank angle θ b, thus completing the preliminary acceleration.

Further, at this moment, the preliminary acceleration stroke becomes 1aand the drawing stroke becomes 1b, which are know values though beingvariable in accordance with the work W to be formed.

In the formation of the work W, the preliminary acceleration stroke 1aand the throttle stroke 1b are first inputted as represented by step 1in the flowchart of FIG. 3.

In the control unit body 14, the calculation processing unit 18calculates the preliminary acceleration starting angle θ a and finishingangle θ b in the step 2 in accordance with the data regarding thestrokes 1a and 1b.

Namely, supposing that the slide speed at the formation time is v, the vis shown as

    u=f'v (θ, spm)=spm·fv(θ)

in which spm represents a stroke/min.

θ=fe (l) and fv (θ) is a function relating to a speed at 1 spm. From theabove, the preliminary acceleration starting angle θ a and theacceleration finishing angle θ b will be calculated as follows.

    la=(1/k) v·t

    t=k·la/spm·fv (θ b)                (1)

    t=60 sec/spm×(θ b-θ a)/360°

    θa=θ b-6·spm·t               (2)

When the equation (1) is applied to the equation (2), ##EQU1##

Accordingly, θ b=f l (l b) and

    θa=θb-k·6·l a/fv (θ b)

In the above equations, the symbol k (1<k) is a coefficient and thedie-cushion is pin-touched at the speed of 1/k of the slide loweringspeed (the upper mold abuts against the blank holder).

According to the preliminary acceleration starting and finishig angles θa and θ b, caluculated above, a table representing the stroke positonsof the die-cushion stroke with respect to the crank angle ispreliminarily prepared and the table is stored in the memory RAM.

Thereafter, when the pressing working is started, in response to thelowering of the slide 10, the crank angle is inputted into the controlunit body 14 by the crank angle detector 12 and the slide speed is alsoinputted therein by the press speed detector 13 (step 3). In the step 4,the calculation processing unit 18 of the control unit body 14discriminates θ a≦θ≦θ b, and in this discrimination, in the case of YES,the process advances to the step 5 to read out the die-cushion strokedata position every crank angle from the table stored in the memory RAMand then to output the data to the comparator 19 as the aimed position.

In the meantime, when the crank angle reaches the preliminaryacceleration starting angle, the servo valve 22 is opened in response tothe servo starting instruction signal so that the pressurized oil flowsin the upper chamber 7, of the hydraulic cylinder 7 and the die-cushionpad 5 starts to the preliminary acceleration (lowering). At this moment,the acceleration stroke of the die-cushion pad 5 is inputted from thedie-cushion stroke position detector 11 into the comparator 19, in whichit is compared with the aimed position outputted from the calculationprocessing unit 18, thereby calculating the deviation therebetween.

Then, the servo valve 22 is subjected to the feedback control so thatthe deviation becomes zero (0).

Accordingly, the die-cushion pad 5 can be accelerated with precisetiming always in accordance with the aimed position and the upper mold23 abuts against the blank holder 2a at the position when thedie-cushion pad 5 lowers with the preliminary acceleration stroke 1b,then finishing the preliminary acceleration.

Thereafter, the slide 10 continues to lower and the work W is thensubjected to the drawing working between the upper and lower molds 23and 2. When the slide reaches the lower dead point, the formation hasbeen completed and the slide 10 raises by the actuation of the pneumaticcylinder 6, thus the die-cushion pad 5 also starting to raise.

Further, as occasion demands, the raising of the die-cushion pad 5 canbe locked by the actuation of the hydraulic cylinder 7 when the servovalve 22 is closed.

A second embodiment of the present invention will be described hereunderwith reference to FIGS. 4 and 5.

Further, the construction near the die-cushion 4 in the secondembiodiment is substantially the same with that of the first embodiment,so that the detailed description thereof is omitted herein by adding thesame reference numerals for avoiding duplication.

Referring to FIG. 4, the die-cushion 4 comprises the pneumatic cylinder6 elastically supporting the die-cushion pad 5 and the hydrauliccylinder 7. The pressure inside the pneumatic cylinder 6 is detected bythe pressure detector 24 and the detected value is transferred to adisplay unit 25 and, on the while, the pressure inside the hydrauliccylinder 7 is detected by the pressure detector 26 and the detectedvalue is inputted to the comparator 34 of a pressure control unit body27.

A pressure control unit comprises this control unit body 27, a pressureinstruction generator 28 for inputting a pressure instruction voltage tothe control unit body 27 and a programable controller 29, and adie-cushion capacity pattern is inputted into the pressure instructiongenerator 28 from a capacity setting panel 30.

Further, the rotation angle (crank angle) of the main shaft of the pressmachine, not shown, is inputted into the pressure instruction generator28 through a rotary encoder 31, and a rotation angle of the main motor,not shown, is inputted into the programable controller 29 through therotation speed detector 32.

The pressure control unit body 27 includes a multiplicator 35 and anintegrator 36 both connected to the output side of the comparator 34. Anoutput from the integrator 36 is outputted from a comparator 37, to aservo motor 40 through a limiter 39 after the correction by a correctedvalue from a multiplicator 38, described hereinlater, to thereby controla hydraulic pressure to be drained to a tank from a pressure chamber 7₃of the hydraulic cylinder 7 through the servo motor 40.

Next, the operation will be described. The factors for the non-linearcontrol object reside in the change of the slide angle in response tothe crank angle and the change of the slide speed, for example, to 1-14SPM (slide/min.) in response to the operation speed.

In consideration of this matter, according to the present invention, thecorrection can be performed by inputting the lowering speed of the slideand the operation speed of the press into the control unit body 27.

Namely, in response to the crank angle inputted from the rotary encoder31 to the pressure instruction generator 28, the slide speed withrespect to each crank angle is read out from the speed tablepreliminarily set in the slide speed table and then D/A converted andinputted into the multiplicator 38.

The press speed is inputted from the programable controller 29 to themultiplicator 38 in response to the signal from the rotation detector 32detecting the rotation of the main motor, and the correction value iscalculated by the multiplicator 38 and then outputted to the comparator37.

On the other hand, the control unit body 27 controls the servo motor 4θalong the line A of FIG. 5 in accordance with the pressure instructionbased on the pressure instruction voltage from the pressure instructiongenerator 28.

Namely, the upper mold 23 starts to lower from the upper dead point ofthe slide, and when the crank angle becomes 120°, for example, the uppermold 23 contacts the blank holder 5 through the work W, and then thepressure is applied to the die-cushion 4. At this time, the servo motor40 is controlled so that a pressure is generated along the pressureinstruction curve A in the pressure chamber 7₃ of the hydraulic cylinder7.

Thereafter, according to the progress of the formation, the pressure inthe pressure chamber 7₃ of the hydraulic cylinder 7 is drained to thetank through the servo valve 40 to thereby maintain constant thepressure in the pressure chamber 7₃ of the hydraulic cylinder 7 and alsoto correct the control signal to be outputted to the servo motor inproportion to the slide lowering speed in accordance with the correctionvalue (α) which has been inputted into the comparator 37 from themultiplicator 38.

That is, when the slide lowering speed is small, the correction is madeto close the servo valve 40, whereas when the slide lowering speedbecomes large, the correction is made to open the servo valve 40.

At the same time, the press operation speed outputted from theprogramable controller 29 is inputted into the multiplicator 35 tothereby correct a gain of the integrator 36.

Namely, when the slide lowering speed is small, the correction is madeto also make small the gain, whereas when the slide lowering speedbecomes large, the correction is made to also make large the gain.

According to the above operation, when the slide lowers to a positionnear the lower dead point to make small the slide lowering speed, theservo valve 40 is controlled to be closed and also to make small thegain of the integrator 36, so that any dullness of an actual pressure issubstantially not found at a portion near the lower dead point and hencethe actual pressure B' can be controlled along the pressure instructioncurve A.

For the sake of confirmation, in the use of a conventional pressurecontrol apparatus, there was observed 10-15% dullness with respect tothe pressure instruction, but according to the pressure controlapparatus of the present invention, the dullness could be reduced lowerthan 2%.

I claim:
 1. A die-cushion apparatus for a press machine including apneumatic cylinder attaining a cushioning function and a hydrauliccylinder enabling and locking cushioning capability, the die-cushionapparatus comprising a pressurized oil makeup circuit for making up apressurized oil to said hydraulic cylinder at a preliminary accelerationperiod of a die-cushion following a lowering motion of a slide, a servovalve disposed to said pressurized oil makeup circuit, and a controlunit body for calculating a crank angle for preliminary accelerationstarting and a crank angle for preliminary acceleration finishing inresponse to a preliminary acceleration stroke value, a drawing strokevalue and a press operation speed value inputted through an operationpanel, for comparing an aimed value of the stroke position of thedie-cushion inputted into a comparator with respect to the respectivecrank angles outputted from a crank angle detector with data relating adie-cushion stroke outputted from a die-cushion stroke position detectorand for controlling said servo valve in a feed back mode so as toeliminate deviation when the deviation is caused between the aimed valueand the data.
 2. A die-cushion apparatus for a press machine comprisinga pneumatic cylinder attaining a cushioning function, a hydrauliccylinder having a pressure discharge line connected to a dischargepressure controlling servo valve to enable and lock a cushioningcapability, a pressure detector disposed to the pressure discharge linebetween the hydraulic cylinder and the servo valve for detecting anactual pressure of the hydraulic cylinder, and a pressure control unitfor comparing in calculation the actual pressure detected by saidpressure detector and a pressure instruction value generated from apressure generator and for controlling said servo valve so as to makecoincident the actual pressure with the pressure instruction value inresponse to the deviation between the actual pressure and the pressureinstruction value, said die-cushion apparatus further comprising meansfor detecting a lowering speed of a slide of a press and a operationspeed of the press and outputting signals representing the detectedlowering speed and the operation speed of the press and means forcorrecting an output signal from said pressure control unit forcontrolling said servo valve in response to the detected signals.